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From "Yonik Seeley (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-977) internal hashing improvements
Date Sat, 11 Aug 2007 13:49:43 GMT
internal hashing improvements
-----------------------------

                 Key: LUCENE-977
                 URL: https://issues.apache.org/jira/browse/LUCENE-977
             Project: Lucene - Java
          Issue Type: Improvement
            Reporter: Yonik Seeley


Internal power-of-two closed hashtable traversal in DocumentsWriter and CharArraySet could
be better.

Here is the current method of resolving collisions:
    if (text2 != null && !equals(text, len, text2)) {
      final int inc = code*1347|1;
      do {
        code += inc;
        pos = code & mask;
        text2 = entries[pos];
      } while (text2 != null && !equals(text, len, text2));

The problem is that two different hashCodes with the same lower bits will keep picking the
same slots (the upper bits will be ignored).
This is because multiplication (*1347) only really shifts bits to the left... so given that
the two codes already matched on the right, they will both pick the same increment, and this
will keep them on the same path through the table (even though it's being added to numbers
that differ on the left).  To resolve this, some bits need to be moved to the right when calculating
the increment.



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